/usr/share/doc/brltty/README-ATB is in brltty 5.0-2ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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Files with names of the form *.atb are attributes tables, and with names of the
form *.ati are attributes subtables. They are used when BRLTTY is displaying
screen attributes rather than screen content. Each of the eight braille dots
represents one of the eight VGA attribute bits.
An attributes table consists of a sequence of directives, one per line, which
define how combinations of VGA attributes are to be represented in braille.
UTF-8 character encoding must be used. White-space (blanks, tabs) at the
beginning of a line, as well as before and/or after any operand of any
directive, is ignored. Lines containing only white-space are ignored. If the
first non-white-space character of a line is "#" then that line is a comment
and is ignored.
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The Dot Directive
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dot <dot> <state> # <comment>
Use the "dot" directive to specify what a specific dot represents. The default
is that all dots are down and not used to represent anything.
The <dot> operand specifies the dot being defined. It is a single digit within
the range 1-8 as defined by the standard braille dot numbering convention (see
README-DOTS).
The <state> operand specifies what the dot represents. It may be:
* =attribute The dot is raised if the named attribute is on.
* ~attribute The dot is raised if the named attribute is off.
The names of the attribute bits are:
* 0X01 fg-blue
* 0X02 fg-green
* 0X04 fg-red
* 0X08 fg-bright
* 0X10 bg-blue
* 0X20 bg-green
* 0X40 bg-red
* 0X80 blink
Examples:
dot 1 =fg-red
dot 2 ~bg-blue
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The Include Directive
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include <file> # <comment>
Use the "include" directive to include the content of an attributes subtable.
It is recursive, which means that any attributes subtable can itself include
yet another attributes subtable. Care must be taken to ensure that an "include
loop" is not created.
The <file> operand specifies the file to be included. It may be either a
relative or an absolute path. If relative, it is anchored at the directory
containing the including file.
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